Electronic ballasts for discharge lamps have proven to be very successful in recent times owing to their known advantages such as increased luminous efficiency and improved efficiency. Conventional low-pressure discharge lamps have heating filaments in the form of electrodes which are heated prior to lamp starting (so-called preheating) in order to increase their emission capacity and therefore to increase the willingness of the lamp to start. In order to ensure effective heating of the filaments, various circuit variants have proven successful in the prior art.
Before the lamp can be operated, it needs to be started by a relatively high voltage. For this purpose, resonance excitation of the lamp resonant circuit is used in many cases. In the case of discharge lamps with preheatable electrodes, the electrodes are first preheated for a specific time before the actual starting voltage is applied. The preheating time is in this case determined by a lamp filament heating controller, which in many cases damps the resonant circuit and/or sets the resonant circuit to off-resonance and therefore keeps the voltage low, with at the same time a current flowing through the lamp filaments.
A simple and extended variant of ensuring preheating is a circuit in which a PTC thermistor is connected in parallel with the lamp and with the resonant capacitor. If the electronic ballast is switched on and the inverter starts up, the PTC thermistor will initially be conducting and allows a preheating current to flow through the electrodes. Owing to the preheating current, the PTC thermistor itself also heats up and ultimately assumes a high resistance value. Therefore, the resonant circuit, which is responsible for the lamp starting, is now only weakly damped and a starting voltage, which is sufficient for starting the lamp, can be built up.
An improved variant of this circuit is specified in DE 41 29 430 A1. Here, the resonant capacitor is split in two, and the PTC thermistor is connected in parallel with one part of the resonant capacitor. This has the advantage that the frequency is closer to the resonant frequency during preheating and therefore a higher preheating current flows, which reduces the preheating time.
Both circuits have the disadvantage that a current flows through the PTC thermistor throughout the entire operating time in order to heat said PTC thermistor such that it keeps a high resistance value. Therefore, it causes a power loss of approximately 0.5-1 watt, which reduces the efficiency of the entire electronic ballast. A further disadvantage is the long cooling-down time of the PTC thermistor, which entails insufficient preheating of the lamp electrodes when the lamp is briefly switched off.
DE 44 25 859 A1 has disclosed an improved circuit which does not have these disadvantages. This circuit simulates a PTC thermistor without having the disadvantage thereof with respect to the high power loss. In principle, in this case, the PTC thermistor is simulated by a transistor (Q3) with a series resistor (Z). During the preheating, the transistor is switched on and represents a resistive load, which drives a preheating current through the lamp electrodes. At the end of the preheating period, the transistor is disconnected in order thus to initiate starting. In the disconnected state, no relevant losses occur. This circuit does not have the disadvantage of insufficient preheating in the event of a brief interruption to the lamp operation either. However, this circuit, as can be seen from the figures of the document, is relatively complex in terms of construction and is therefore cost-intensive.